Cable networks have, in recent years, moved beyond merely broadcasting television signals over a coaxial cable to subscribers in their homes. Subscribers of a cable network nowadays have a modem allowing transmission of digital signals upstream toward a headend of the network. Among many services afforded by cable modems are: an Internet service, a home shopping service using a television catalogue, and a voice-over-IP (VoIP) phone service.
In bidirectional cable networks, upstream and downstream signals occupy separate frequency bands called upstream and downstream frequency bands. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 860 MHz. Downstream information channel signals co-propagate in the downstream spectral band, and upstream signals co-propagate in the upstream spectral band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals propagating in a common cable in opposite directions.
To provide upstream communication capability to a multitude of subscribers, the upstream frequency channels are used in a so-called time-division multiplexing (TDM) mode. Each cable modem is assigned a time slot, within which it is allowed to transmit information in form of short-duration radio-frequency (RF) bursts. The time slots to transmit the RF bursts by individual modems are assigned dynamically by a cable modem termination system (CMTS) disposed at the headend of the cable network.
The terminal devices, such as TV receivers and cable modems, are installed in customer premises, and thus are not easily accessible by cable network personnel. Electrical noise results from electromagnetic interference at the customer premises, “ringing” and ground loops due to improper equipment installation, faulty or damaged cabling, etc. The noise propagates back along the return paths of the signal towards the headend. Noise originating at a single location of a cable network can impede or even block upstream communications not only for a cable modem at that location, but also for other cable modems of the network. To make matters worse, noise from various locations of a cable network can accumulate as it propagates upstream towards the headend, increasing in magnitude due to a “funneling” effect of the upstream paths converging to a single point at the headend. This accumulated upstream path noise, commonly termed “ingress noise”, represents a constant challenge for cable network operators. Not infrequently, isolating and eliminating a source of the ingress noise takes a major part of a network cable technician's workday.
To isolate a noise source, the technician measures noise levels at each input of a “bridger” amplifier, to determine which input exhibits the highest level of noise. The technician then proceeds to a next downstream amplifier connected to the noisiest input of the first amplifier, and repeats the measurement to isolate a noisiest input of the downstream amplifier. In going from amplifier to amplifier, the technician travels to various locations in the field, repeating the measurements until the source of the ingress noise is finally located. This iterative process, called “noise segmentation”, can take up to 70% and more of the technician's work time. Not surprisingly, troubleshooting the ingress noise represents a major cost driver for delivery of two-way services by cable operators.
A supplementary troubleshooting method, which is sometimes used to the dismay of a technician's supervisor, is to disconnect power to all amplifiers downstream of the amplifier being tested, by removing a corresponding power jumper or fuse. If the noise is being introduced further downstream than the next active element, it will disappear when power is no longer supplied to that element; if the noise is being introduced somewhere between the amplifier under test and the next downstream amplifier, the noise will remain. By using this method, the technician can save one test per troubleshooting operation, obtaining approximately a 15 to 45-minute time savings per operation. However, each time the fuse is pulled, all services are disrupted to all homes in the portion of the network served by the downstream active elements. Due to the disruptive nature of the fuse-pulling method, network service technicians are discouraged from using it, despite the time savings achievable.
Various methods of less intrusive, automated ingress noise localization have been suggested. By way of example, Reichert in U.S. Pat. No. 4,520,508 discloses an ingress noise monitor disposed at a remote node. The noise monitor measures a noise level at the node and provides information about the measured noise level by amplitude-modulating the return path signal. Detrimentally, the Reichert devices are comparatively complex and costly. Installation of autonomous noise meters of Reichert across a node having dozens of legs can be prohibitively expensive.
Sanders et al. in U.S. Pat. Nos. 5,742,713 and 5,737,461 disclose a remotely or locally controllable upstream ingress filter disposed at a node. The filter is switched ON by pulling down a DC voltage applied to the cable. When the voltage is pulled, the filter short-circuits a low frequency band containing the upstream signal, for ingress diagnostic purposes. By pulling the voltage while observing a change of noise strength at the headend, the noise source can be localized. Detrimentally, the method of Sanders et al. requires independent and complex DC voltage control of all the cables extending from the fiber node to individual actives, which can be difficult to do when the cables running to different nodes are powered in series.
High cost of test equipment and maintenance associated with ingress noise segmentation have caused some providers to increase the robustness of cable network with respect to the ingress noise, so that the ingress noise segmentation would not be required. By way of example, Masuda et al. in U.S. Pat. No. 6,868,552 assigned to Fujitsu Limited of Kawasaki, Japan, discloses an ingress noise blocking device in form of a gate switch located at a node and switchable ON only during upstream RF bursts. This is done to suppress all signals, including the ingress noise, between the RF bursts. Since the ingress noise between the bursts is suppressed, it cannot impede reception of upstream RF bursts from other nodes, that occur at different moments of time. Thus, the “funneling effect” of the ingress noise in a cable network is reduced.
Similarly, Baran et al. in U.S. Pat. Nos. 6,049,693 and 6,094,211 disclose a remotely operable ingress noise blocking filter placed at a terminating junction of a cable network. The filter is configured to suppress upstream signals, allowing the upstream signals to pass only when a control signal is received from a downstream cable modem during short durations when the cable modem is allowed to transmit a signal.
Of course, the systems of Baran and Masuda do not address a case where the ingress noise impedes the reception of the RF bursts from the very leg where the blocking filter is installed, especially when the RF bursts on multiple legs occur at the same time but at different carrier frequencies.
The prior art is lacking a simple, inexpensive, yet widely deployable solution for ingress noise localization. It is a goal of the invention to provide such a solution.